A stick-shaped ignition coil includes a coil portion, a control portion, and a tower portion. The coil portion is located in an axially intermediate position of the ignition coil. The control portion is located on the upper end side of the ignition coil. The tower portion is located on the lower end side of the ignition coil. The tower portion and the coil portion are inserted into a plughole formed in a cylinder head of the engine. The control portion protrudes from the upper plane of the cylinder head. The diameters of both the coil portion and the tower portion are determined to be slightly smaller than the inner diameter of the plughole, so that the coil portion and the tower portion can be easily inserted into the plughole. Specifically, the diameters of both the coil portion and the tower portion are within a range from 20 mm to 22 mm. The diameter of the control portion is within a range from 23 mm to 25 mm. Therefore, an annular gap is formed between the coil portion and the plughole, and another annular gap is formed between the tower portion and the plughole. The coil portion and the tower portion may be radially vibrated within the annular gap due to vibration of the engine. Accordingly, a radially positioning portion needs to be provided to the ignition coil to reduce vibration of the outer circumferential periphery of the coil portion with respect to the inner circumferential periphery of the plughole.
As shown in FIG. 7, a conventional ignition coil for an internal combustion engine, which is disclosed in JP-B2-3393770, includes a coil portion 100 that has a cylindrical resinous case 101 receiving primary and secondary coils (not shown). An upper positioning protrusion 103 is formed in the vicinity of the upper end portion of the cylindrical resinous case 101. A lower positioning protrusion 104 is formed in the vicinity of the lower end portion of the cylindrical resinous case 101. Both the upper and lower positioning protrusions 103, 104 respectively protrude radially outwardly from the outer circumferential periphery of the resinous case 101. Tip ends of the upper and lower positioning protrusions 103, 104 respectively make contact with the inner periphery of the plughole 106 in the radial direction thereof. Alternatively, the tip ends of the upper and lower positioning protrusions 103, 104 are respectively in proximity radially to the inner periphery of the plughole 106. Thus, the coil portion 100 is protected from vibrating in the plughole 106.
However, the outer diameter of a coil portion of an ignition coil needs to be decreased, because of decrease of the inner diameter of the plughole in recent years. Therefore, the resinous case 101 is reduced from the conventional coil portion 100, and a thin film sheet is provided to the ignition coil instead of the resinous case 101. The thin film sheet is circumferentially covered with a metallic outer core that is exposed to the outermost periphery of the coil portion. In this structure, a positioning member may not be easily formed on the outer circumferential periphery of the metallic outer core. Accordingly, the coil portion may not be properly positioned in the plughole.